Roll-up doors are used in commercial or industrial establishments to separate different areas of a building or to separate the inside of the building from the exterior. For example, roll-up doors may be used to separate two areas or zones which have different temperature or humidity conditions, or to provide noise control between two areas. A roll-up door has the advantage of being capable of being moved rapidly between the open and closed positions, and when in the open position, the door panel or curtain is wound on a drum located above the doorway so that the door panel will not obstruct the doorway.
The typical roll-up door includes a counterbalancing mechanism to counterbalance the weight of the door panel, and also incorporates a tensioning system which is arranged to exert a downward force on the door panel to stretch the door panel when in the closed or partially closed positions.
The conventional roll-up door also includes a horizontal beam that is attached to the lower edge of the flexible door panel. The operation of the roll-up door is normally initiated by the operator of a forklift truck, either by the operator pulling on a suspended cable, or by the forklift truck passing over a floor treadle. In the case of a power failure, or in a situation where the forklift operator may not fully actuate or pull the cable, or in the event that the forklift operator does not properly judge the speed of the forklift truck with the opening rate of the door, the forklift truck may engage the door panel and beam with substantial impact. The impact can bend or deform the beam, or rip the fabric door panel away from the beam, or damage the guide mechanism in the side frames of the door.
Because of this problem, it has been proposed to incorporate a breakaway mechanism with the beam of a roll-up door and one such breakaway mechanism is that described in U S. patent application 07/679,770, filed Apr. 3, 1991, U.S. Pat. No. 5,139,074. With the breakaway mechanism of that patent application, the beam is formed of a flexible material, and each end of the beam is formed with an open-ended slot or recess. Guide brackets are mounted for vertical sliding movement within guide tracks in the side frames of the door, and each guide bracket is provided with an extension which is freely slidable in the slot in the end of the beam. With the flexible nature of the beam, a minor bump against the beam will merely flex the beam, and move the guide extensions longitudinally in the slot without releasing the guide extensions from the slots. On the other hand, if the beam is subjected to a substantial external transverse impact, the beam will initially deflect and continued deformation of the beam will release the guide extensions from the beam ends.
Another typical breakaway mechanism includes a T-shaped guide bracket extension, and the beam end has a mating T-shaped slot. With this construction, the beam will move transversely when subjected to an external impact to thereby release the connection of the beam ends to the guide brackets.
The tensioning mechanism utilized in a conventional roll-up door exerts a downward biasing force on the guide brackets, and when the beam breaks away from the guide brackets, the biasing force will drive the guide brackets downwardly within the guide tracks. As the biasing force is of substantial magnitude, the guide brackets or other components can be damaged. Moreover, it is an extremely difficult task to reconnect the guide brackets with the ends of the beam. More particularly, a workman must raise the guide bracket manually against the biasing force to an elevation corresponding to the level of the beam and then, while overcoming the biasing force, must reattach the guide brackets to the beam ends. In some situations, a workman is not able to manually overcome the biasing force, with the result that the biasing force must be relieved before reattachment can be made.
Because of the problems encountered in reattaching the releasable beam of the roll-up door, there has been a need for a quick and simple manner of reattachment.